<article id="wikiArticle">
<div></div>
<p> <strong><code>handler.has()</code></strong> 方法可以看作是针对 <a href="Reference/Operators/in" title="如果指定的属性在指定的对象或其原型链中，则in 运算符返回true。"><code>in</code></a> 操作的钩子.</p>
<h2 id="Syntax">Syntax</h2>
<pre><code  class="language-javascript">var p = new Proxy(target, {
  has: function(target, prop) {
  }
});
</code></pre>
<h3 id="Parameters">Parameters</h3>
<p><code>下面是传递给 has</code> 方法的参数. <code>this</code> is bound to the handler.</p>
<dl>
<dt><code>target</code></dt>
<dd>目标对象.</dd>
<dt><code>prop</code></dt>
<dd>需要检查是否存在的属性.</dd>
</dl>
<h3 id="Return_value">Return value</h3>
<p><code>has</code> 方法返回一个 boolean 属性的值.</p>
<h2 id="Description">Description</h2>
<p><code><strong>handler.has</strong></code> 方法可以看作是针对 <a href="Reference/Operators/in" title="如果指定的属性在指定的对象或其原型链中，则in 运算符返回true。"><code>in</code></a> 操作的钩子.</p>
<h3 id="Interceptions">Interceptions</h3>
<p>这个钩子可以拦截下面这些操作:</p>
<ul>
<li>属性查询: <code>foo in proxy</code></li>
<li>继承属性查询: <code>foo in Object.create(proxy)</code></li>
<li><code>with</code> 检查<code>: with(proxy) { (foo); }</code></li>
<li><a href="Reference/Global_Objects/Reflect/has" title="静态方法 Reflect.has() 作用与 in 操作符 相同。"><code>Reflect.has()</code></a></li>
</ul>
<h3 id="Invariants">Invariants</h3>
<p>如果违反了下面这些规则,  proxy 将会抛出 <a href="Reference/Global_Objects/TypeError" title="TypeError（类型错误） 对象用来表示值的类型非预期类型时发生的错误。"><code>TypeError</code></a>:</p>
<ul>
<li>如果目标对象的某一属性本身不可被配置，则该属性不能够被代理隐藏.</li>
<li>如果目标对象为不可扩展对象，则该对象的属性不能够被代理隐藏</li>
</ul>
<h2 id="Examples">Examples</h2>
<p>下面的代码 hook 了 <a href="Reference/Operators/in" title="如果指定的属性在指定的对象或其原型链中，则in 运算符返回true。"><code>in</code></a> 操作.</p>
<pre><code  class="language-javascript">var p = new Proxy({}, {
  has: function(target, prop) {
    console.log('called: ' + prop);
    return true;
  }
});

console.log('a' in p); // "called: a"
                       // true
</code></pre>
<p>下面的代码违反了规则.</p>
<pre><code  class="language-javascript">var obj = { a: 10 };
Object.preventExtensions(obj);
var p = new Proxy(obj, {
  has: function(target, prop) {
    return false;
  }
});

'a' in p; // TypeError is thrown
</code></pre>
<h2 id="Specifications">Specifications</h2>
<table class="standard-table">
<tbody>
<tr>
<th scope="col">Specification</th>
<th scope="col">Status</th>
<th scope="col">Comment</th>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/ecma-262/6.0/#sec-proxy-object-internal-methods-and-internal-slots-hasproperty-p" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">[[HasProperty]]</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td>Initial definition.</td>
</tr>
<tr>
<td><a class="external" href="https://tc39.github.io/ecma262/#sec-proxy-object-internal-methods-and-internal-slots-hasproperty-p" hreflang="en" lang="en" rel="noopener">ECMAScript Latest Draft (ECMA-262)<br/><small lang="zh-CN">[[HasProperty]]</small></a></td>
<td><span class="spec-Draft">Draft</span></td>
<td> </td>
</tr>
</tbody>
</table>
<h2 id="Browser_compatibility">Browser compatibility</h2>
<div><div class="blockIndicator warning"><strong><a class="external" href="https://github.com/mdn/browser-compat-data" rel="noopener">We're converting our compatibility data into a machine-readable JSON format</a></strong>.
            This compatibility table still uses the old format,
            because we haven't yet converted the data it contains.
            <strong><a class="new" href="/zh-CN/docs/MDN/Contribute/Structures/Compatibility_tables" rel="nofollow">Find out how you can help!</a></strong></div>
<div class="htab">
<a id="AutoCompatibilityTable" name="AutoCompatibilityTable"></a>
<ul>
<li class="selected"><a>Desktop</a></li>
<li><a>Mobile</a></li>
</ul>
</div></div>
<div id="compat-desktop">
<table class="compat-table">
<tbody>
<tr>
<th>Feature</th>
<th>Chrome</th>
<th>Firefox (Gecko)</th>
<th>Internet Explorer</th>
<th>Opera</th>
<th>Safari</th>
</tr>
<tr>
<td>Basic support</td>
<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
<td><a href="/en-US/Firefox/Releases/18" title="Released on 2013-01-08.">18</a> (18)</td>
<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
</tr>
</tbody>
</table>
</div>
<div id="compat-mobile">
<table class="compat-table">
<tbody>
<tr>
<th>Feature</th>
<th>Android</th>
<th>Chrome for Android</th>
<th>Firefox Mobile (Gecko)</th>
<th>IE Mobile</th>
<th>Opera Mobile</th>
<th>Safari Mobile</th>
</tr>
<tr>
<td>Basic support</td>
<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
<td>18.0 (18)</td>
<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
<td><span style="color: rgb(255, 153, 0);" title="Compatibility unknown; please update this.">?</span></td>
</tr>
</tbody>
</table>
</div>
<h2 id="See_also">See also</h2>
<ul>
<li><a href="Reference/Global_Objects/Proxy" title="Proxy 对象用于定义基本操作的自定义行为（如属性查找，赋值，枚举，函数调用等）。"><code>Proxy</code></a></li>
<li><a href="Reference/Global_Objects/Proxy/handler" title="处理器对象用来自定义代理对象的各种可代理操作。"><code>handler</code></a></li>
<li><a href="Reference/Operators/in" title="如果指定的属性在指定的对象或其原型链中，则in 运算符返回true。"><code>in</code></a> operator</li>
<li><a href="Reference/Global_Objects/Reflect/has" title="静态方法 Reflect.has() 作用与 in 操作符 相同。"><code>Reflect.has()</code></a></li>
</ul>
</article>